U.S. Pat. No. 5,211,024, assigned to the assignee hereof, discloses an apparatus for purification of refrigerant that includes a refrigerant pump and a filter/dryer unit for removing water from refrigerant pumped therethrough. A flow meter is coupled to the filter/dryer unit for measuring volumetric flow of refrigerant passing through the filter/dryer unit. Volumetric flow of refrigerant through the flow meter is monitored over time for indicating to an operator when the filter/dryer unit should be replaced as a function of mass of refrigerant pumped through the filter/dryer unit and independent of types of refrigerant pumped through the filter/dryer unit. Operation of the refrigerant pump may be inhibited upon indication that the filter/dryer unit should be replaced.
Although the apparatus disclosed in the noted patent addresses and overcomes problems theretofore extant in the art, further improvements remain desirable. For example, the cost of the refrigerant volumetric flow meter is significant, and can limit market acceptance of the apparatus. Moisture concentration and density is assumed to be at the highest expected operating temperature. In addition, filter capacity is wasted for refrigerants whose product of moisture concentration times density is lower than that of other refrigerants. It is therefore an object of the present invention to provide a method and apparatus of the subject character that can be manufactured at reduced cost, and that take into account both refrigerant type and refrigerant temperature/pressure effects on both moisture concentration and density so that the filter/dryer water absorption capacity can be fully utilized for all types of refrigerant and under an enhanced range of operating conditions.
Apparatus for purification of refrigerant in accordance with a presently preferred embodiment of the invention comprises a replaceable filter/dryer unit for removing water from refrigerant passing therethrough, and a refrigerant pump for pumping refrigerant through the filter/dryer unit. At least one predetermined parameter (e.g., pressure or temperature) of the refrigerant flowing through the filter/dryer unit is measured, and actual mass flow rate of refrigerant pumped through the filter/dryer unit is determined as a function of such measured parameter. The mass flow rate of refrigerant so determined is monitored as a function of time of operation of the refrigerant pump, and filter status is indicated to an operator and/or operation of the pump is terminated when need for a filter/dryer change is indicated. The invention thus improves upon the above-noted patent by obtaining an actual measure of refrigerant mass flow rate, rather than measuring volumetric flow rate and estimating mass flow rate while assuming specific refrigerant pressure and temperature conditions.
Actual mass flow rate of water in the refrigerant pumped through the filter/dryer unit preferably is determined as a function of volumetric pumping speed of the pump, which is a constant for a given pump displacement and shaft speed, and water concentration per unit mass of refrigerant which is a function of the measured refrigerant parameter. The mass flow rate of water in refrigerant pumped through the filter/dryer is determined as a function of the pumping speed multiplied by the water concentration per unit mass of refrigerant. Preferably, the relationship of water concentration per unit mass of refrigerant is stored as look-up tables in a microprocessor as a function of the measured refrigerant parameter. Preferably, the refrigerant pump comprises a compressor that pumps refrigerant through the filter/dryer unit in vapor phase, and the measured refrigerant parameter comprises compressor inlet suction pressure. Compressor inlet vapor pressure is more indicative of refrigerant density during final stages of refrigerant recovery when the compressor draws a vacuum on the equipment under service to meet EPA requirements and saturated conditions.
For purification of plural differing refrigerants having differing density and moisture solubility characteristics, the apparatus of the present invention preferably includes facility for identifying and distinguishing among differing refrigerant types. This may be determined, for example, as a function of vapor pressure and temperature of refrigerant fed to the inlet of the compressor/pump. Pumping speed in units of mass per unit time and water concentration per unit mass of refrigerant are determined as a combined function of refrigerant vapor pressure and refrigerant type, preferably by means of look-up tables in a control microprocessor.